Probe heads used by coordinate-measuring instruments for measuring the three-dimensional coordinates of selected points of a workpiece can be divided into two categories, namely, into those of the "measuring" type and those of the so-called "switching" type. Probe heads of the measuring type produce a signal which is proportional to the deflection of their probe or probe pin; on the other hand, probe heads of the switching type produce a pulselike signal at the instant of probe contact with the workpiece which is to be measured. These switching-probe heads are generally so constructed that a biasing element urges a seating member (to which the probe pin is chucked) into a mount on the fixed part of the probe head, and the seating of this member in the mount unambiguously determines the zero position of the probe pin. In the work-contacting process, the probe deflects and lifts the seating member out of its mount, against the force of the biasing element, and the seated relation resumes upon loss of work contact, i.e., at termination of the contacting process. In known switching probes, the mount usually comprises a three-point support, wherein the seating member of the probe has three angularly spaced cylindrical arms, and wherein each of these arms has static seating engagement in a V-groove formed by a pair of balls in the mount, i.e., within the housing of the probe head. Probe heads having such a statically determined mounting are described, inter alia, in U.S. Pat. No. 4,153,998.
In some of the known probe heads, electrical contacts are provided in the seating locations, for the production of a work-contact signal However, it is also known to generate the contact pulse via a separate piezoelectric sensor. A probe head having such a sensor is described, for example, in U.S. Pat. No. 4,177,568.
In probe heads of the switching type, the precision with which the probe returns to its seated or zero position (i.e., after a work-contacting process) is to a considerable extent limited by friction in the seating of the probe in its mount. To reduce this friction, the seating locations are customarily greased or oiled. However, there remains a residual error by which the workcontact ball of the probe pin can deviate from its zero position. This error depends also on the length of the probe pin, the effective diameter of the seating base in the mount, and the biasing force which acts in the seating direction. In commercial probes, this seating error has values in the range of 0.2 to 0.5 micrometer.
Such a seating error can also occur, although to a lesser extent, in measuring-type probe heads, since the probe pin in a measuring-type probe head is also in some way mechanically form-locked in its zero position.
Greater deviations from the zero position can also occur sporadically. The case may even arise that electrical contacts in a switching-type probe head remain open. And it has been proposed to remedy such rare occurrences of error, by employing a device which applies a mechanical push to the probe pin when a seating contact has remained open. However, only the few such occurrences (which happen perhaps once in several thousand scanning processes) are eliminated by these measures, which are described in Federal Republic of Germany OS 3,623,614. General improvement in the precision of seated zero positioning thus cannot be obtained in this way.
Federal Republic of Germany AS 2,841,424 discloses a probe head having a piezoelectric oscillator which permits the free end of the probe to oscillate continuously with a small amplitude. This known probe head is so operated that a second, separate transducer detects the amplitude of the oscillation. It is then possible to recognize the condition of contact by the second transducer and to produce a corresponding signal, since the amplitude of probe oscillation is disturbed or damped as long as there is probe contact with the workpiece.
The problem of bearing friction and lack of restoring precision is not addressed in this German patent, it being merely indicated that a measurement precision of .+-.1 micrometer is achievable for this known probe head. However, for highly precise applications in coordinate measurement procedures, this measurement of uncertainty of .+-.1 micrometer is not sufficient.